Modified starches have been used for years in many diverse applications. There is a continuing need to improve upon starch modification processes and modified starches to meet various demands of these applications.
One conventional method for modifying starch, hydroxypropylation, involves the addition of sodium sulfate and sodium hydroxide in addition to propylene oxide to a slurry of native or modified starch. Sodium hydroxide raises the pH of the slurry to about 11.5, thereby facilitating the hydroxypropylation reaction between the starch and the propylene oxide. Hydroxypropylated starch, in a high pH environment, will gelatinize, so sodium sulfate is added to the starch slurry, for example, at a level of about 10% or more on a dry basis, in order to inhibit gelatinization of the hydroxypropylated starch. After hydroxypropylation has been completed, the pH of the slurry is reduced by adding an acid, and the resulting hydroxypropylated modified starch is washed and dried for further use. A schematic diagram of this conventional process is shown in FIG. 1.
The modified starch resulting from the conventional hydroxypropylation reaction contains sodium salt at a level of about 15-25% by dry weight of the modified starch before the washing step. The washing step typically reduces the sodium salt to a level of about 0.5% or less on a dry basis.
Another conventional method for modifying starch is by cross-linking the starch. This can be done by adding cross-linking agents, including, but not limited to, a mixture of sodium trimetaphosphate (STMP) and sodium tripolyphosphate (STPP), or phosphorus oxychloride and epichlorohydrin, to either a native starch slurry or starch slurry that has previously been hydroxypropylated. About 5% sodium chloride is added to the reaction in order to modulate the cross-linking process to provide uniformity and consistency in this typically fast reaction. The resulting modified starch contains sodium salt at a level of about 5-25% by dry weight (before the washing step).